The term “oil shale” refers to a sedimentary rock interspersed with an organic mixture of complex chemical compounds collectively referred to as “kerogen.” The oil shale consists of laminated sedimentary rock containing mainly clay with fine sand, calcite, dolomite, and iron compounds. Oil shales can vary in their mineral and chemical composition. When the oil shale is heated to above 250-400° F., destructive distillation of the kerogen occurs to produce products in the form of oil, gas, and residual carbon. The hydrocarbonaceous products resulting from the destructive distillation of the kerogen have uses which are similar to petroleum products. Indeed, oil shale is considered to be one of the primary sources for producing liquid fuels and natural gas to supplement and augment those fuels currently produced from petroleum sources.
Processes for recovering hydrocarbonaceous products from oil shale may generally be divided into in situ processes and above-ground processes. In situ processes involve treating oil shale which is still in the ground in order to remove the hydrocarbonaceous products, while above-ground processes require removing the oil shale from the ground through mining procedures and then subsequently retorting in above-ground equipment. Clearly, in situ processes are economically and environmentally desirable since removal of the oil shale from the ground is often expensive and destructive. However, in situ processes are generally not as efficient as above-ground processes in terms of total product recovery.
Historically, prior art in situ processes have generally only been concerned with recovering products from oil shale which comes to the surface of the ground; thus, prior art processes have typically not been capable of recovering products from oil shale located at great depths below the ground surface. For example, typical prior art in situ processes generally only treat oil shale which is 300 feet or less below the ground surface. However, many oil shale deposits extend far beyond the 300 foot depth level; in fact, oil shale deposits of 3000 feet or more deep are not uncommon.
Moreover, many, if not most, prior art processes are directed towards recovering products from what is known as the “mahogany” layer of the oil shale. The mahogany layer is the richest zone of the oil shale bed, having a Fischer assay of about twenty-five gallons per ton (25 gal/ton) or greater. The Mahogany Zone in the Piceance Creek Basin consists of kerogen-rich strata and averages 100 to 200 ft thick. This layer has often been the only portion of the oil shale bed to which many prior art in situ processes have been applied.
For economic reasons, it has been found generally uneconomical in the prior art to recover products from any other area of the oil shale bed than the mahogany zone.
Thus, there exists a relatively untapped resource of oil shale, especially deep-lying oil shale and oil shale outside of the mahogany zone, which have not been treated by prior art processes mainly due to the absence of an economically viable method for recovering products from such oil shale.
Another important disadvantage of many, if not most prior art in situ oil shale processes is that expensive rubilization procedures are often necessary before treating the oil shale. Rubilization of the in situ oil shale formation is typically accomplished by triggering underground explosions so as to break up the oil shale formation. In such prior art process, it has been necessary to rubilize the oil shale formation so as to provide a substantial increase in the permeability of the oil shale bearing rock formation. By increasing the permeability, the ability for gases and liquids to flow also increases, the potential to recover a more substantial portion of products therefrom. However, rubilization procedures are expensive, time-consuming, and often cause the ground surface to recede so as to significantly destroy the structural integrity of the underground formation and the terrain supported thereby. This destruction of the structural integrity of the ground and surrounding terrain is a source of great environmental concern.
Rubilization of the oil shale in prior art in situ processes has a further disadvantage. Upon destructive distillation of the kerogen in the oil shale to produce various hydrocarbonaceous products, these products seek the path of least resistance when escaping through the marlstone of the oil shale formation. By rubilizing the oil shale formation, many different paths of escape are created for the products; the result is that it is difficult to predict the path which the products will follow. This, of course, is important in terms of withdrawing the products from the rubilized oil shale formation so as to enable maximum recovery of the products. Since the products have numerous possible escape paths to follow within the rubilized oil shale formation, the task of recovering the products is greatly complicated and significant sub surface environmental issues become more of an issue. Including significant groundwater contamination.
Oil/tar sands, often referred to as ‘extra heavy oil,’ are types of bitumen deposits. The deposits are naturally occurring mixtures of sand or clay, water and an extremely dense and viscous form of petroleum called bitumen. They are found in large amounts in many countries throughout the world, but are found in extremely large quantities in the United States, Canada and Venezuela.
Due to the fact that extra-heavy oil and bitumen flow very slowly, if at all, toward producing wells under normal reservoir conditions, the sands are often extracted by strip mining or the oil made to flow into wells by in situ techniques which reduce the viscosity by injecting steam, solvents, and/or hot air into the sands. These processes can use more water and require larger amounts of energy than conventional oil extraction, although many conventional oil fields also require large amounts of water and energy to achieve good rates of production.
Certain improvements with respect to the recovery of products from shale are disclosed in U.S. Pat. No. 4,928,765. Unlike other prior art processes, the in situ body of oil shale to be treated is not rubilized. Rather, a gas-fired heater assembly is placed within a bore hole followed by the introduction of fuel gas and combustion air from above ground, both of which are regulated to maintain an initial start-up temperature of over 1000° F. and thereafter a constant temperature of below 1500° F. throughout a reaction zone formed in the surrounding shale bed. Specifically, a production temperature of 1200° F. was been found most desirable. By maintenance of this temperature, voids created in the reaction zone as kerogen is retorted to evolve natural gas, become black body radiators assisting to ensure a sustained, constant high volume extraction of natural gas devoid of any liquids.
Like all mining and non-renewable resource development projects, oil shale and sands operations have an effect on the environment. Oil sands projects may affect the land when the bitumen is initially mined and with large deposits of toxic chemicals, the water during the separation process and through the drainage of rivers, and the air due to the release of carbon dioxide and other emissions, as well as deforestation. Clearly any improvements in the techniques use to extract hydrocarbonaceous products from shale and sands would be appreciated, particularly if efficiency is improved and/or environmental impact is reduced.